Surely all Marvel fans of the Silver Age remember this classic 1968 cover...
X-Men 50 by Steranko, which introduced Lorna Dane, aka Polaris, to the team. Not only that, she was possibly the daughter of Magneto as well, something that seemed more alarming before we found out that Pietro and Wanda were his children.
I love this cover, with the composition and the green color scheme radiating out of Polaris. Not only that--this issue featured a brand new logo for the X-Men, designed by Steranko himself. One of my favorite logos of all time!
Mike Mayhew did a cover recreation of this over on his deviantArt page. I think it is a wonderful homage to Steranko with great use of Mayhew's color style. Nuff Said!